I have an in-ground spa that is approx 1600 gallons. It is a salt system and has 3M colored plaster (tan color). I drained the water yesterday and refilled the spa. The strange thing is that wherever there was standing water (i.e. puddles on top of steps or water dripping down the wall to the seat then down to the floor), remained as darker areas than the rest of the spa once it finally refilled. I didn't do any acid washing or chemical treatments. Simply drained and refilled. So the water that remained (areas that didn't dry completely during the refill) was the same water that actually dried in other places. This makes no sense to me.